The present invention is concerned with apparatus for filling and sealing alkali metal electrode containers for alkali metal energy conversion devices. This invention also relates to a method of filling alkali metal electrode containers for alkali metal energy conversion devices with alkali metal.
Such devices employ an alkali metal as one or sometimes both of the electrodes in a cell including a separator, or electrolyte, between the electrodes, formed of a solid ceramic material which is electronically insulating but conductive to cations of the alkali metal.
A particular example of such devices is the sodium sulphur cell which employs liquid sodium as the anode of the cell and liquid sulphur/sodium polysulphide material as the cathode, the two being separated by a solid electrolyte of beta alumina.
Sodium sulphur cells for experimental purposes have been made in small quantities and it is very readily possible to fill the sodium reservoir or container of the cell by carrying out the filling operation in a closed chamber filled with a suitable inert gas, the cell being sealed before removal from the chamber. The entire operation is carried out at a temperature substantially in excess of the melting point of sodium, typically 150.degree. C.
However, such a technique is not suitable for quantity production of cells. Since the cells must be heated within the chamber prior to filling and then allowed to cool again. Furthermore, once the filled cells have cooled, the chamber must be broached to remove the cells and for the insertion of a fresh batch, whereupon the atmosphere in the chamber must be purged of any air before the next batch is heated and filled.
GB-A-2061598 describes a method and apparatus for filling sodium sulphur cells which avoids some of these difficulties. In the described arrangement, each sodium container or reservoir is provided with an open ended metal filler tube protruding from the container. Empty containers are heated progressively and presented to a sodium filling head whilst in air at atmospheric pressure. The filling head is specially designed to seal around the filler tube of the sodium container, evacuate air from within the sodium container before feeding sodium into the evacuated container. The filling head is then designed to pinch off the metal tube to seal the container before breaking the seal made with the filler tube. To facilitate this operation, it is usually necessary for the filler head to have a feed tube which can be inserted down through the filler tube of the cell into the interior of the sodium container so as to feed sodium directly into the container. The requirement for the filling head to seal to the filler tube of the sodium container also renders the design of the filling head relatively complicated as can be appreciated from the above referred specification.